On Fri, Sep 29, 2017 at 10:54 AM, Peter Geoghegan <p...@bowt.ie> wrote:
>> Something that allocates new memory as the patch's bloom_init()
>> function does I'd tend to call 'make' or 'create' or 'new' or
>> something, rather than 'init'.
>
> I tend to agree. I'll adopt that style in the next version. I just
> didn't want the caller to have to manage the memory themselves.
v3 of the patch series, attached, does it that way -- it adds a
bloom_create(). The new bloom_create() function still allocates its
own memory, but does so while using a FLEXIBLE_ARRAY_MEMBER. A
separate bloom_init() function (that works with dynamic shared memory)
could easily be added later, for the benefit of parallel hash join.
Other notable changes:
* We now support bloom filters that have bitsets of up to 512MB in
size. The previous limit was 128MB.
* We now use TransactionXmin for our AccessShareLock xmin cutoff,
rather than calling GetOldestXmin(). This is the same cut-off used by
xacts that must avoid broken hot chains for their earliest snapshot.
This avoids a scan of the proc array, and allows more thorough
verification, since GetOldestXmin() was overly restrictive here.
* Expanded code comments describing the kinds of problems the new
verification capability is expected to be good at catching.
For example, there is now a passing reference to the CREATE INDEX
CONCURRENTLY bug that was fixed back in February (it's given as an
example of a more general problem -- faulty HOT safety assessment).
With the new heapallindexed enhancement added by this patch, amcheck
can be expected to catch that issue much of the time. We also go into
heap-only tuple handling within IndexBuildHeapScan(). The way that
CREATE INDEX tries to index the most recent tuple in a HOT chain
(while locating the root tuple in the chain, to get the right heap TID
for the index) has proven to be very useful as a smoke test while
investigating HOT/VACUUM FREEZE bugs in the past couple of weeks [1].
I believe it would have caught several historic MultiXact/recovery
bugs, too. This all seems worth noting explicitly.
[1]
https://postgr.es/m/cah2-wznm4rcrhfaiwkpwtpew2bxdtgrozk7jwwgucxeh3d1...@mail.gmail.com
--
Peter Geoghegan
From 3bed03a9e0506c0b81097b634c5f1b5534a2dcb3 Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <p...@bowt.ie>
Date: Tue, 2 May 2017 00:19:24 -0700
Subject: [PATCH 2/2] Add amcheck verification of indexes against heap.
Add a new, optional capability to bt_index_check() and
bt_index_parent_check(): callers can check that each heap tuple that
ought to have an index entry does in fact have one. This happens at the
end of the existing verification checks.
This is implemented by using a Bloom filter data structure. The
implementation performs set membership tests within a callback (the same
type of callback that each index AM registers for CREATE INDEX). The
Bloom filter is populated during the initial index verification scan.
---
contrib/amcheck/Makefile | 2 +-
contrib/amcheck/amcheck--1.0--1.1.sql | 28 ++++
contrib/amcheck/amcheck.control | 2 +-
contrib/amcheck/expected/check_btree.out | 14 +-
contrib/amcheck/sql/check_btree.sql | 9 +-
contrib/amcheck/verify_nbtree.c | 275 ++++++++++++++++++++++++++++---
doc/src/sgml/amcheck.sgml | 157 ++++++++++++++----
src/include/utils/snapmgr.h | 2 +-
8 files changed, 423 insertions(+), 66 deletions(-)
create mode 100644 contrib/amcheck/amcheck--1.0--1.1.sql
diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index 43bed91..c5764b5 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -4,7 +4,7 @@ MODULE_big = amcheck
OBJS = verify_nbtree.o $(WIN32RES)
EXTENSION = amcheck
-DATA = amcheck--1.0.sql
+DATA = amcheck--1.0--1.1.sql amcheck--1.0.sql
PGFILEDESC = "amcheck - function for verifying relation integrity"
REGRESS = check check_btree
diff --git a/contrib/amcheck/amcheck--1.0--1.1.sql b/contrib/amcheck/amcheck--1.0--1.1.sql
new file mode 100644
index 0000000..e6cca0a
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.0--1.1.sql
@@ -0,0 +1,28 @@
+/* contrib/amcheck/amcheck--1.0--1.1.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.1'" to load this file. \quit
+
+--
+-- bt_index_check()
+--
+DROP FUNCTION bt_index_check(regclass);
+CREATE FUNCTION bt_index_check(index regclass,
+ heapallindexed boolean DEFAULT false)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+--
+-- bt_index_parent_check()
+--
+DROP FUNCTION bt_index_parent_check(regclass);
+CREATE FUNCTION bt_index_parent_check(index regclass,
+ heapallindexed boolean DEFAULT false)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want these to be available to public
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index 05e2861..4690484 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
# amcheck extension
comment = 'functions for verifying relation integrity'
-default_version = '1.0'
+default_version = '1.1'
module_pathname = '$libdir/amcheck'
relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index df3741e..42872b8 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -16,8 +16,8 @@ RESET ROLE;
-- we, intentionally, don't check relation permissions - it's useful
-- to run this cluster-wide with a restricted account, and as tested
-- above explicit permission has to be granted for that.
-GRANT EXECUTE ON FUNCTION bt_index_check(regclass) TO bttest_role;
-GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass) TO bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass, boolean) TO bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass, boolean) TO bttest_role;
SET ROLE bttest_role;
SELECT bt_index_check('bttest_a_idx');
bt_index_check
@@ -56,8 +56,14 @@ SELECT bt_index_check('bttest_a_idx');
(1 row)
--- more expansive test
-SELECT bt_index_parent_check('bttest_b_idx');
+-- more expansive tests
+SELECT bt_index_check('bttest_a_idx', true);
+ bt_index_check
+----------------
+
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true);
bt_index_parent_check
-----------------------
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index fd90531..5d27969 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -19,8 +19,8 @@ RESET ROLE;
-- we, intentionally, don't check relation permissions - it's useful
-- to run this cluster-wide with a restricted account, and as tested
-- above explicit permission has to be granted for that.
-GRANT EXECUTE ON FUNCTION bt_index_check(regclass) TO bttest_role;
-GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass) TO bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass, boolean) TO bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass, boolean) TO bttest_role;
SET ROLE bttest_role;
SELECT bt_index_check('bttest_a_idx');
SELECT bt_index_parent_check('bttest_a_idx');
@@ -42,8 +42,9 @@ ROLLBACK;
-- normal check outside of xact
SELECT bt_index_check('bttest_a_idx');
--- more expansive test
-SELECT bt_index_parent_check('bttest_b_idx');
+-- more expansive tests
+SELECT bt_index_check('bttest_a_idx', true);
+SELECT bt_index_parent_check('bttest_b_idx', true);
BEGIN;
SELECT bt_index_check('bttest_a_idx');
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 868c14e..f73ea31 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -8,6 +8,11 @@
* (the insertion scankey sort-wise NULL semantics are needed for
* verification).
*
+ * When index-to-heap verification is requested, a Bloom filter is used to
+ * fingerprint all tuples in the target index, as the index is traversed to
+ * verify its structure. A heap scan later verifies the presence in the heap
+ * of all index tuples fingerprinted within the Bloom filter.
+ *
*
* Copyright (c) 2017, PostgreSQL Global Development Group
*
@@ -18,11 +23,13 @@
*/
#include "postgres.h"
+#include "access/htup_details.h"
#include "access/nbtree.h"
#include "access/transam.h"
#include "catalog/index.h"
#include "catalog/pg_am.h"
#include "commands/tablecmds.h"
+#include "lib/bloomfilter.h"
#include "miscadmin.h"
#include "storage/lmgr.h"
#include "utils/memutils.h"
@@ -53,10 +60,13 @@ typedef struct BtreeCheckState
* Unchanging state, established at start of verification:
*/
- /* B-Tree Index Relation */
+ /* B-Tree Index Relation and associated heap relation */
Relation rel;
+ Relation heaprel;
/* ShareLock held on heap/index, rather than AccessShareLock? */
bool readonly;
+ /* Also verifying heap has no unindexed tuples? */
+ bool heapallindexed;
/* Per-page context */
MemoryContext targetcontext;
/* Buffer access strategy */
@@ -72,6 +82,15 @@ typedef struct BtreeCheckState
BlockNumber targetblock;
/* Target page's LSN */
XLogRecPtr targetlsn;
+
+ /*
+ * Mutable state, for optional heapallindexed verification:
+ */
+
+ /* Bloom filter fingerprints B-Tree index */
+ bloom_filter *filter;
+ /* Debug counter */
+ int64 heaptuplespresent;
} BtreeCheckState;
/*
@@ -92,15 +111,20 @@ typedef struct BtreeLevel
PG_FUNCTION_INFO_V1(bt_index_check);
PG_FUNCTION_INFO_V1(bt_index_parent_check);
-static void bt_index_check_internal(Oid indrelid, bool parentcheck);
+static void bt_index_check_internal(Oid indrelid, bool parentcheck,
+ bool heapallindexed);
static inline void btree_index_checkable(Relation rel);
-static void bt_check_every_level(Relation rel, bool readonly);
+static void bt_check_every_level(Relation rel, Relation heaprel,
+ bool readonly, bool heapallindexed);
static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
BtreeLevel level);
static void bt_target_page_check(BtreeCheckState *state);
static ScanKey bt_right_page_check_scankey(BtreeCheckState *state);
static void bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
ScanKey targetkey);
+static void bt_tuple_present_callback(Relation index, HeapTuple htup,
+ Datum *values, bool *isnull,
+ bool tupleIsAlive, void *checkstate);
static inline bool offset_is_negative_infinity(BTPageOpaque opaque,
OffsetNumber offset);
static inline bool invariant_leq_offset(BtreeCheckState *state,
@@ -116,37 +140,47 @@ static inline bool invariant_leq_nontarget_offset(BtreeCheckState *state,
static Page palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum);
/*
- * bt_index_check(index regclass)
+ * bt_index_check(index regclass, heapallindexed boolean)
*
* Verify integrity of B-Tree index.
*
* Acquires AccessShareLock on heap & index relations. Does not consider
- * invariants that exist between parent/child pages.
+ * invariants that exist between parent/child pages. Optionally verifies
+ * that heap does not contain any unindexed or incorrectly indexed tuples.
*/
Datum
bt_index_check(PG_FUNCTION_ARGS)
{
Oid indrelid = PG_GETARG_OID(0);
+ bool heapallindexed = false;
- bt_index_check_internal(indrelid, false);
+ if (PG_NARGS() == 2)
+ heapallindexed = PG_GETARG_BOOL(1);
+
+ bt_index_check_internal(indrelid, false, heapallindexed);
PG_RETURN_VOID();
}
/*
- * bt_index_parent_check(index regclass)
+ * bt_index_parent_check(index regclass, heapallindexed boolean)
*
* Verify integrity of B-Tree index.
*
* Acquires ShareLock on heap & index relations. Verifies that downlinks in
- * parent pages are valid lower bounds on child pages.
+ * parent pages are valid lower bounds on child pages. Optionally verifies
+ * that heap does not contain any unindexed or incorrectly indexed tuples.
*/
Datum
bt_index_parent_check(PG_FUNCTION_ARGS)
{
Oid indrelid = PG_GETARG_OID(0);
+ bool heapallindexed = false;
- bt_index_check_internal(indrelid, true);
+ if (PG_NARGS() == 2)
+ heapallindexed = PG_GETARG_BOOL(1);
+
+ bt_index_check_internal(indrelid, true, heapallindexed);
PG_RETURN_VOID();
}
@@ -155,7 +189,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
* Helper for bt_index_[parent_]check, coordinating the bulk of the work.
*/
static void
-bt_index_check_internal(Oid indrelid, bool parentcheck)
+bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed)
{
Oid heapid;
Relation indrel;
@@ -191,9 +225,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck)
/*
* Since we did the IndexGetRelation call above without any lock, it's
* barely possible that a race against an index drop/recreation could have
- * netted us the wrong table. Although the table itself won't actually be
- * examined during verification currently, a recheck still seems like a
- * good idea.
+ * netted us the wrong table.
*/
if (heaprel == NULL || heapid != IndexGetRelation(indrelid, false))
ereport(ERROR,
@@ -204,8 +236,8 @@ bt_index_check_internal(Oid indrelid, bool parentcheck)
/* Relation suitable for checking as B-Tree? */
btree_index_checkable(indrel);
- /* Check index */
- bt_check_every_level(indrel, parentcheck);
+ /* Check index, possibly against table it is an index on */
+ bt_check_every_level(indrel, heaprel, parentcheck, heapallindexed);
/*
* Release locks early. That's ok here because nothing in the called
@@ -253,11 +285,14 @@ btree_index_checkable(Relation rel)
/*
* Main entry point for B-Tree SQL-callable functions. Walks the B-Tree in
- * logical order, verifying invariants as it goes.
+ * logical order, verifying invariants as it goes. Optionally, verification
+ * checks if the heap relation contains any tuples that are not represented in
+ * the index but should be.
*
* It is the caller's responsibility to acquire appropriate heavyweight lock on
* the index relation, and advise us if extra checks are safe when a ShareLock
- * is held.
+ * is held. (A lock of the same type must also have been acquired on the heap
+ * relation.)
*
* A ShareLock is generally assumed to prevent any kind of physical
* modification to the index structure, including modifications that VACUUM may
@@ -272,7 +307,8 @@ btree_index_checkable(Relation rel)
* parent/child check cannot be affected.)
*/
static void
-bt_check_every_level(Relation rel, bool readonly)
+bt_check_every_level(Relation rel, Relation heaprel, bool readonly,
+ bool heapallindexed)
{
BtreeCheckState *state;
Page metapage;
@@ -283,15 +319,35 @@ bt_check_every_level(Relation rel, bool readonly)
/*
* RecentGlobalXmin assertion matches index_getnext_tid(). See note on
* RecentGlobalXmin/B-Tree page deletion.
+ *
+ * We also rely on TransactionXmin having been initialized by now.
*/
Assert(TransactionIdIsValid(RecentGlobalXmin));
+ Assert(TransactionIdIsNormal(TransactionXmin));
/*
* Initialize state for entire verification operation
*/
state = palloc(sizeof(BtreeCheckState));
state->rel = rel;
+ state->heaprel = heaprel;
state->readonly = readonly;
+ state->heapallindexed = heapallindexed;
+
+ if (state->heapallindexed)
+ {
+ int64 total_elems;
+ uint32 seed;
+
+ /* Size Bloom filter based on estimated number of tuples in index */
+ total_elems = (int64) state->rel->rd_rel->reltuples;
+ /* Random seed relies on backend srandom() call to avoid repetition */
+ seed = random();
+ /* Create Bloom filter to fingerprint index */
+ state->filter = bloom_create(total_elems, maintenance_work_mem, seed);
+ state->heaptuplespresent = 0;
+ }
+
/* Create context for page */
state->targetcontext = AllocSetContextCreate(CurrentMemoryContext,
"amcheck context",
@@ -347,6 +403,45 @@ bt_check_every_level(Relation rel, bool readonly)
previouslevel = current.level;
}
+ /*
+ * * Heap contains unindexed/malformed tuples check *
+ */
+ if (state->heapallindexed)
+ {
+ IndexInfo *indexinfo;
+
+ if (state->readonly)
+ elog(DEBUG1, "verifying presence of all required tuples in index \"%s\"",
+ RelationGetRelationName(rel));
+ else
+ elog(DEBUG1, "verifying presence of required tuples in index \"%s\" with xmin before %u",
+ RelationGetRelationName(rel), TransactionXmin);
+
+ indexinfo = BuildIndexInfo(state->rel);
+
+ /*
+ * Since we're not actually indexing, don't enforce uniqueness/wait for
+ * concurrent insertion to finish, even with unique indexes.
+ *
+ * Force use of MVCC snapshot (reuse CONCURRENTLY infrastructure) when
+ * only an AccessShareLock held. It seems like a good idea to not
+ * diverge from expected heap lock strength in all cases. This is
+ * needed to prevent unhelpful WARNINGs due to concurrent insertions
+ * that IndexBuildHeapScan() does not expect.
+ */
+ indexinfo->ii_Unique = false;
+ indexinfo->ii_Concurrent = !state->readonly;
+ IndexBuildHeapScan(state->heaprel, state->rel, indexinfo, true,
+ bt_tuple_present_callback, (void *) state);
+
+ ereport(DEBUG1,
+ (errmsg_internal("finished verifying presence of " INT64_FORMAT " tuples (proportion of bits set: %f) from table \"%s\"",
+ state->heaptuplespresent, bloom_prop_bits_set(state->filter),
+ RelationGetRelationName(heaprel))));
+
+ bloom_free(state->filter);
+ }
+
/* Be tidy: */
MemoryContextDelete(state->targetcontext);
}
@@ -499,7 +594,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
errdetail_internal("Block pointed to=%u expected level=%u level in pointed to block=%u.",
current, level.level, opaque->btpo.level)));
- /* Verify invariants for page -- all important checks occur here */
+ /* Verify invariants for page */
bt_target_page_check(state);
nextpage:
@@ -546,6 +641,9 @@ nextpage:
*
* - That all child pages respect downlinks lower bound.
*
+ * This is also where heapallindexed callers use their Bloom filter to
+ * fingerprint IndexTuples.
+ *
* Note: Memory allocated in this routine is expected to be released by caller
* resetting state->targetcontext.
*/
@@ -589,6 +687,11 @@ bt_target_page_check(BtreeCheckState *state)
itup = (IndexTuple) PageGetItem(state->target, itemid);
skey = _bt_mkscankey(state->rel, itup);
+ /* Fingerprint leaf page tuples (those that point to the heap) */
+ if (state->heapallindexed && P_ISLEAF(topaque) && !ItemIdIsDead(itemid))
+ bloom_add_element(state->filter, (unsigned char *) itup,
+ IndexTupleSize(itup));
+
/*
* * High key check *
*
@@ -682,8 +785,10 @@ bt_target_page_check(BtreeCheckState *state)
* * Last item check *
*
* Check last item against next/right page's first data item's when
- * last item on page is reached. This additional check can detect
- * transposed pages.
+ * last item on page is reached. This additional check will detect
+ * transposed pages iff the supposed right sibling page happens to
+ * belong before target in the key space. (Otherwise, a subsequent
+ * heap verification will probably detect the problem.)
*
* This check is similar to the item order check that will have
* already been performed for every other "real" item on target page
@@ -1062,6 +1167,134 @@ bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
}
/*
+ * Per-tuple callback from IndexBuildHeapScan, used to determine if index has
+ * all the entries that definitely should have been observed in leaf pages of
+ * the target index (that is, all IndexTuples that were fingerprinted by our
+ * Bloom filter). All heapallindexed checks occur here.
+ *
+ * Theory of operation:
+ *
+ * The redundancy between an index and the table it indexes provides a good
+ * opportunity to detect corruption, especially corruption within the table.
+ * The high level principle behind the verification performed here is that any
+ * IndexTuple that should be in an index following a fresh CREATE INDEX (based
+ * on the same index definition) should also have been in the original,
+ * existing index, which should have used exactly the same representation
+ * (Index tuple formation is assumed to be deterministic, and IndexTuples are
+ * assumed immutable; while the LP_DEAD bit is mutable, that's ItemId metadata,
+ * which is not fingerprinted). There will often be some dead-to-everyone
+ * IndexTuples fingerprinted by the Bloom filter, but we only try to detect the
+ * *absence of needed tuples*, so that's okay.
+ *
+ * Since the overall structure of the index has already been verified, the most
+ * likely explanation for error here is a corrupt heap page (could be logical
+ * or physical corruption). Index corruption may still be detected here,
+ * though. Only readonly callers will have verified that left links and right
+ * links are in agreement, and so it's possible that a leaf page transposition
+ * within index is actually the source of corruption detected here (for
+ * !readonly callers). The checks performed only for readonly callers might
+ * more accurately frame the problem as a cross-page invariant issue (this
+ * could even be due to recovery not replaying all WAL records). The !readonly
+ * ERROR message raised here includes a HINT about retrying with readonly
+ * verification, just in case it's a cross-page invariant issue, though that
+ * doesn't particularly likely.
+ *
+ * IndexBuildHeapScan() expects to be able to find the root tuple when a
+ * heap-only tuple (the live tuple at the end of some HOT chain) needs to be
+ * indexed, in order to replace the actual tuple's TID with the root tuple's
+ * TID (which is what we're actually passed back here). The index build heap
+ * scan code will raise an error when a tuple that claims to be the root of the
+ * heap-only tuple's HOT chain cannot be located. This catches cases where the
+ * original root item offset/root tuple for a HOT chain indicates (for whatever
+ * reason) that the entire HOT chain is dead, despite the fact that the latest
+ * heap-only tuple should be indexed. When this happens, sequential scans may
+ * always give correct answers, and all indexes may be considered structurally
+ * consistent (i.e. the nbtree structural checks would not detect corruption).
+ * It may be the case that only index scans give wrong answers, and yet heap or
+ * SLRU corruption is the real culprit. (While it's true that LP_DEAD bit
+ * setting will probably also leave the index in a corrupt state before too
+ * long, the problem is nonetheless that there is heap corruption.)
+ *
+ * Note also that heap-only tuple handling within IndexBuildHeapScan() detects
+ * index tuples that contain the wrong values. This can happen when there is
+ * no superseding index tuple due to a faulty assessment of HOT safety.
+ * Because the latest tuple's contents are used with the root TID, an error
+ * will be raised when a tuple with the same TID but different (correct)
+ * attribute values is passed back to us. (Faulty assessment of HOT-safety was
+ * behind the CREATE INDEX CONCURRENTLY bug that was fixed in February of
+ * 2017.)
+ */
+static void
+bt_tuple_present_callback(Relation index, HeapTuple htup, Datum *values,
+ bool *isnull, bool tupleIsAlive, void *checkstate)
+{
+ BtreeCheckState *state = (BtreeCheckState *) checkstate;
+ IndexTuple itup;
+
+ Assert(state->heapallindexed);
+
+ /* Must recheck visibility when only AccessShareLock held */
+ if (!state->readonly)
+ {
+ TransactionId xmin;
+
+ /*
+ * Don't test for presence in index where xmin not at least old enough
+ * that we know for sure that absence of index tuple wasn't just due to
+ * some transaction performing insertion after our verifying index
+ * traversal began. (Actually, the cut-off used is a point where
+ * preceding write transactions must have committed/aborted. We should
+ * have already fingerprinted all index tuples for all such preceding
+ * transactions, because the cut-off was established before our index
+ * traversal even began.)
+ *
+ * You might think that the fact that an MVCC snapshot is used by the
+ * heap scan (due to our indicating that this is the first scan of a
+ * CREATE INDEX CONCURRENTLY index build) would make this test
+ * redundant. That's not quite true, because with current
+ * IndexBuildHeapScan() interface caller cannot do the MVCC snapshot
+ * acquisition itself. In this way, heap tuple coverage is similar to
+ * the coverage we could get by using the existing transaction
+ * snapshot. It's easier to do this than to adopt the
+ * IndexBuildHeapScan() interface to our narrow requirements.
+ */
+ Assert(tupleIsAlive);
+ xmin = HeapTupleHeaderGetXmin(htup->t_data);
+ if (!TransactionIdPrecedes(xmin, TransactionXmin))
+ return;
+ }
+
+ /*
+ * Generate an index tuple.
+ *
+ * Note that we rely on deterministic index_form_tuple() TOAST compression.
+ * If index_form_tuple() was ever enhanced to compress datums out-of-line,
+ * or otherwise varied when or how compression was applied, our assumption
+ * would break, leading to false positive reports of corruption. For now,
+ * we don't decompress/normalize toasted values as part of fingerprinting.
+ */
+ itup = index_form_tuple(RelationGetDescr(index), values, isnull);
+ itup->t_tid = htup->t_self;
+
+ /* Probe Bloom filter -- tuple should be present */
+ if (bloom_lacks_element(state->filter, (unsigned char *) itup,
+ IndexTupleSize(itup)))
+ ereport(ERROR,
+ (errcode(ERRCODE_DATA_CORRUPTED),
+ errmsg("heap tuple (%u,%u) from table \"%s\" lacks matching index tuple within index \"%s\"",
+ ItemPointerGetBlockNumber(&(itup->t_tid)),
+ ItemPointerGetOffsetNumber(&(itup->t_tid)),
+ RelationGetRelationName(state->heaprel),
+ RelationGetRelationName(state->rel)),
+ !state->readonly
+ ? errhint("Retrying verification using the function bt_index_parent_check() might provide a more specific error.")
+ : 0));
+
+ state->heaptuplespresent++;
+ pfree(itup);
+}
+
+/*
* Is particular offset within page (whose special state is passed by caller)
* the page negative-infinity item?
*
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index dd71dbd..48bdb13 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -44,7 +44,7 @@
<variablelist>
<varlistentry>
<term>
- <function>bt_index_check(index regclass) returns void</function>
+ <function>bt_index_check(index regclass, heapallindexed boolean DEFAULT false) returns void</function>
<indexterm>
<primary>bt_index_check</primary>
</indexterm>
@@ -55,7 +55,7 @@
<function>bt_index_check</function> tests that its target, a
B-Tree index, respects a variety of invariants. Example usage:
<screen>
-test=# SELECT bt_index_check(c.oid), c.relname, c.relpages
+test=# SELECT bt_index_check(index => c.oid, heapallindexed => i.indisprimary)
FROM pg_index i
JOIN pg_opclass op ON i.indclass[0] = op.oid
JOIN pg_am am ON op.opcmethod = am.oid
@@ -83,20 +83,23 @@ ORDER BY c.relpages DESC LIMIT 10;
</screen>
This example shows a session that performs verification of every
catalog index in the database <quote>test</>. Details of just
- the 10 largest indexes verified are displayed. Since no error
- is raised, all indexes tested appear to be logically consistent.
- Naturally, this query could easily be changed to call
+ the 10 largest indexes verified are displayed. Verification of
+ the presence of heap tuples as index tuples is requested for
+ primary key indexes only. Since no error is raised, all indexes
+ tested appear to be logically consistent. Naturally, this query
+ could easily be changed to call
<function>bt_index_check</function> for every index in the
database where verification is supported.
</para>
<para>
- <function>bt_index_check</function> acquires an <literal>AccessShareLock</>
- on the target index and the heap relation it belongs to. This lock mode
- is the same lock mode acquired on relations by simple
- <literal>SELECT</> statements.
+ <function>bt_index_check</function> acquires an
+ <literal>AccessShareLock</> on the target index and the heap
+ relation it belongs to. This lock mode is the same lock mode
+ acquired on relations by simple <literal>SELECT</> statements.
<function>bt_index_check</function> does not verify invariants
- that span child/parent relationships, nor does it verify that
- the target index is consistent with its heap relation. When a
+ that span child/parent relationships, but will verify the
+ presence of all heap tuples as index tuples within the index
+ when <parameter>heapallindexed</> is <literal>true</>. When a
routine, lightweight test for corruption is required in a live
production environment, using
<function>bt_index_check</function> often provides the best
@@ -108,7 +111,7 @@ ORDER BY c.relpages DESC LIMIT 10;
<varlistentry>
<term>
- <function>bt_index_parent_check(index regclass) returns void</function>
+ <function>bt_index_parent_check(index regclass, heapallindexed boolean DEFAULT false) returns void</function>
<indexterm>
<primary>bt_index_parent_check</primary>
</indexterm>
@@ -117,19 +120,22 @@ ORDER BY c.relpages DESC LIMIT 10;
<listitem>
<para>
<function>bt_index_parent_check</function> tests that its
- target, a B-Tree index, respects a variety of invariants. The
- checks performed by <function>bt_index_parent_check</function>
- are a superset of the checks performed by
- <function>bt_index_check</function>.
- <function>bt_index_parent_check</function> can be thought of as
- a more thorough variant of <function>bt_index_check</function>:
- unlike <function>bt_index_check</function>,
+ target, a B-Tree index, respects a variety of invariants.
+ Optionally, when the <parameter>heapallindexed</> argument is
+ <literal>true</>, the function verifies the presence of all heap
+ tuples that should be found within the index. The checks
+ performed by <function>bt_index_parent_check</function> are a
+ superset of the checks performed by
+ <function>bt_index_check</function> when called with the same
+ options. <function>bt_index_parent_check</function> can be
+ thought of as a more thorough variant of
+ <function>bt_index_check</function>: unlike
+ <function>bt_index_check</function>,
<function>bt_index_parent_check</function> also checks
- invariants that span parent/child relationships. However, it
- does not verify that the target index is consistent with its
- heap relation. <function>bt_index_parent_check</function>
- follows the general convention of raising an error if it finds a
- logical inconsistency or other problem.
+ invariants that span parent/child relationships.
+ <function>bt_index_parent_check</function> follows the general
+ convention of raising an error if it finds a logical
+ inconsistency or other problem.
</para>
<para>
A <literal>ShareLock</> is required on the target index by
@@ -159,6 +165,70 @@ ORDER BY c.relpages DESC LIMIT 10;
</sect2>
<sect2>
+ <title>Optional <parameter>heapallindexed</> verification</title>
+ <para>
+ When the <parameter>heapallindexed</> argument to verification
+ functions is <literal>true</>, an additional phase of verification
+ is performed against the table associated with the target index
+ relation. This consists of a <quote>dummy</> <command>CREATE
+ INDEX</> operation, which checks for the presence of all would-be
+ new index tuples against a temporary, in-memory summarizing
+ structure (this is built when needed during the first, standard
+ phase). The summarizing structure <quote>fingerprints</> every
+ tuple found within the target index. The high level principle
+ behind <parameter>heapallindexed</> verification is that a new index
+ that is equivalent to the existing, target index must only have
+ entries that can be found in the existing structure.
+ </para>
+ <para>
+ The additional <parameter>heapallindexed</> phase adds significant
+ overhead: verification will typically take several times longer than
+ it would with only the standard consistency checking of the target
+ index's structure. However, verification will still take
+ significantly less time than an actual <command>CREATE INDEX</>.
+ There is no change to the relation-level locks acquired when
+ <parameter>heapallindexed</> verification is performed. The
+ summarizing structure is bound in size by
+ <varname>maintenance_work_mem</varname>. In order to ensure that
+ there is no more than a 2% probability of failure to detect the
+ absence of any particular index tuple, approximately 2 bytes of
+ memory are needed per index tuple. As less memory is made available
+ per index tuple, the probability of missing an inconsistency
+ increases. This is considered an acceptable trade-off, since it
+ limits the overhead of verification very significantly, while only
+ slightly reducing the probability of detecting a problem, especially
+ for installations where verification is treated as a routine
+ maintenance task.
+ </para>
+ <para>
+ In many applications, even the default
+ <varname>maintenance_work_mem</varname> setting of <literal>64MB</>
+ will be sufficient to have less than a 2% probability of overlooking
+ any single absent or corrupt tuple. This will be the case when
+ there are no indexes with more than about 30 million distinct index
+ tuples, regardless of the overall size of any index, the total
+ number of indexes, or anything else. False positive candidate tuple
+ membership tests within the summarizing structure occur at random,
+ and are very unlikely to be the same for repeat verification
+ operations. Furthermore, within a single verification operation,
+ each missing or malformed index tuple independently has the same
+ chance of being detected. If there is any inconsistency at all, it
+ isn't particularly likely to be limited to a single tuple. All of
+ these factors favor accepting a limited per operation per tuple
+ probability of missing corruption, in order to enable performing
+ more thorough index to heap verification more frequently (practical
+ concerns about the overhead of verification are likely to limit the
+ frequency of verification). In aggregate, the probability of
+ detecting a hardware fault or software defect actually
+ <emphasis>increases</> significantly with this strategy in most real
+ world cases. Moreover, frequent verification allows problems to be
+ caught earlier on average, which helps to limit the overall impact
+ of corruption, and often simplifies root cause analysis.
+ </para>
+
+ </sect2>
+
+ <sect2>
<title>Using <filename>amcheck</> effectively</title>
<para>
@@ -199,17 +269,31 @@ ORDER BY c.relpages DESC LIMIT 10;
</listitem>
<listitem>
<para>
+ Structural inconsistencies between indexes and the heap relations
+ that are indexed (when <parameter>heapallindexed</> verification
+ is performed).
+ </para>
+ <para>
+ There is no cross-checking of indexes against their heap relation
+ during normal operation. Symptoms of heap corruption can be very
+ subtle.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
Corruption caused by hypothetical undiscovered bugs in the
- underlying <productname>PostgreSQL</> access method code or sort
- code.
+ underlying <productname>PostgreSQL</> access method code, sort
+ code, or transaction management code.
</para>
<para>
Automatic verification of the structural integrity of indexes
plays a role in the general testing of new or proposed
<productname>PostgreSQL</> features that could plausibly allow a
- logical inconsistency to be introduced. One obvious testing
- strategy is to call <filename>amcheck</> functions continuously
- when running the standard regression tests. See <xref
+ logical inconsistency to be introduced. Verification of table
+ structure and associated visibility and transaction status
+ information plays a similar role. One obvious testing strategy
+ is to call <filename>amcheck</> functions continuously when
+ running the standard regression tests. See <xref
linkend="regress-run"> for details on running the tests.
</para>
</listitem>
@@ -242,6 +326,13 @@ ORDER BY c.relpages DESC LIMIT 10;
<emphasis>absolute</emphasis> protection against failures that
result in memory corruption.
</para>
+ <para>
+ When <parameter>heapallindexed</> is <literal>true</>, and heap
+ verification is performed, there is generally a greatly increased
+ chance of detecting single-bit errors, since strict binary
+ equality is tested, and the indexed attributes within the heap
+ are tested.
+ </para>
</listitem>
</itemizedlist>
In general, <filename>amcheck</> can only prove the presence of
@@ -253,11 +344,9 @@ ORDER BY c.relpages DESC LIMIT 10;
<title>Repairing corruption</title>
<para>
No error concerning corruption raised by <filename>amcheck</> should
- ever be a false positive. In practice, <filename>amcheck</> is more
- likely to find software bugs than problems with hardware.
- <filename>amcheck</> raises errors in the event of conditions that,
- by definition, should never happen, and so careful analysis of
- <filename>amcheck</> errors is often required.
+ ever be a false positive. <filename>amcheck</> raises errors in the
+ event of conditions that, by definition, should never happen, and so
+ careful analysis of <filename>amcheck</> errors is often required.
</para>
<para>
There is no general method of repairing problems that
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index fc64153..565260f 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -56,7 +56,7 @@ extern TimestampTz GetOldSnapshotThresholdTimestamp(void);
extern bool FirstSnapshotSet;
-extern TransactionId TransactionXmin;
+extern PGDLLIMPORT TransactionId TransactionXmin;
extern TransactionId RecentXmin;
extern PGDLLIMPORT TransactionId RecentGlobalXmin;
extern TransactionId RecentGlobalDataXmin;
--
2.7.4
From 38930ff5cefb2915ff4ce294f3dc275530b4872d Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <p...@bowt.ie>
Date: Thu, 24 Aug 2017 20:58:21 -0700
Subject: [PATCH 1/2] Add Bloom filter data structure implementation.
A Bloom filter is a space-efficient, probabilistic data structure that
can be used to test set membership. Callers will sometimes incur false
positives, but never false negatives. The rate of false positives is a
function of the total number of elements and the amount of memory
available for the Bloom filter.
Two classic applications of Bloom filters are cache filtering, and data
synchronization testing. Any user of Bloom filters must accept the
possibility of false positives as a cost worth paying for the benefit in
space efficiency.
---
src/backend/lib/Makefile | 4 +-
src/backend/lib/README | 2 +
src/backend/lib/bloomfilter.c | 303 ++++++++++++++++++++++++++++++++++++++++++
src/include/lib/bloomfilter.h | 27 ++++
4 files changed, 334 insertions(+), 2 deletions(-)
create mode 100644 src/backend/lib/bloomfilter.c
create mode 100644 src/include/lib/bloomfilter.h
diff --git a/src/backend/lib/Makefile b/src/backend/lib/Makefile
index d1fefe4..191ea9b 100644
--- a/src/backend/lib/Makefile
+++ b/src/backend/lib/Makefile
@@ -12,7 +12,7 @@ subdir = src/backend/lib
top_builddir = ../../..
include $(top_builddir)/src/Makefile.global
-OBJS = binaryheap.o bipartite_match.o dshash.o hyperloglog.o ilist.o \
- knapsack.o pairingheap.o rbtree.o stringinfo.o
+OBJS = binaryheap.o bipartite_match.o bloomfilter.o dshash.o hyperloglog.o \
+ ilist.o knapsack.o pairingheap.o rbtree.o stringinfo.o
include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/lib/README b/src/backend/lib/README
index 5e5ba5e..376ae27 100644
--- a/src/backend/lib/README
+++ b/src/backend/lib/README
@@ -3,6 +3,8 @@ in the backend:
binaryheap.c - a binary heap
+bloomfilter.c - probabilistic, space-efficient set membership testing
+
hyperloglog.c - a streaming cardinality estimator
pairingheap.c - a pairing heap
diff --git a/src/backend/lib/bloomfilter.c b/src/backend/lib/bloomfilter.c
new file mode 100644
index 0000000..7ba44c7
--- /dev/null
+++ b/src/backend/lib/bloomfilter.c
@@ -0,0 +1,303 @@
+/*-------------------------------------------------------------------------
+ *
+ * bloomfilter.c
+ * Minimal Bloom filter
+ *
+ * A Bloom filter is a probabilistic data structure that is used to test an
+ * element's membership of a set. False positives are possible, but false
+ * negatives are not; a test of membership of the set returns either "possibly
+ * in set" or "definitely not in set". This can be very space efficient when
+ * individual elements are larger than a few bytes, because elements are hashed
+ * in order to set bits in the Bloom filter bitset.
+ *
+ * Elements can be added to the set, but not removed. The more elements that
+ * are added, the larger the probability of false positives. Caller must hint
+ * an estimated total size of the set when its Bloom filter is initialized.
+ * This is used to balance the use of memory against the final false positive
+ * rate.
+ *
+ * Copyright (c) 2017, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ * src/backend/lib/bloomfilter.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <math.h>
+
+#include "access/hash.h"
+#include "lib/bloomfilter.h"
+
+#define MAX_HASH_FUNCS 10
+
+typedef struct bloom_filter
+{
+ /* K hash functions are used, which are randomly seeded */
+ int k_hash_funcs;
+ uint32 seed;
+ /* Bitset is sized directly in bits. It must be a power-of-two <= 2^32. */
+ int64 bitset_bits;
+ unsigned char bitset[FLEXIBLE_ARRAY_MEMBER];
+} bloom_filter;
+
+static int my_bloom_power(int64 target_bitset_bits);
+static int optimal_k(int64 bitset_bits, int64 total_elems);
+static void k_hashes(bloom_filter *filter, uint32 *hashes, unsigned char *elem,
+ size_t len);
+static uint32 sdbmhash(unsigned char *elem, size_t len);
+
+/*
+ * Create Bloom filter in caller's memory context. This should get a false
+ * positive rate of between 1% and 2% when bitset is not constrained by memory.
+ *
+ * total_elems is an estimate of the final size of the set. It ought to be
+ * approximately correct, but we can cope well with it being off by perhaps a
+ * factor of five or more. See "Bloom Filters in Probabilistic Verification"
+ * (Dillinger & Manolios, 2004) for details of why this is the case.
+ *
+ * bloom_work_mem is sized in KB, in line with the general work_mem convention.
+ *
+ * The Bloom filter behaves non-deterministically when caller passes a random
+ * seed value. This ensures that the same false positives will not occur from
+ * one run to the next, which is useful to some callers.
+ *
+ * Notes on appropriate use:
+ *
+ * To keep the implementation simple and predictable, the underlying bitset is
+ * always sized as a power-of-two number of bits, and the largest possible
+ * bitset is 512MB. The implementation is therefore well suited to data
+ * synchronization problems between unordered sets, where predictable
+ * performance is more important than worst case guarantees around false
+ * positives. Another problem that the implementation is well suited for is
+ * cache filtering where good performance already relies upon having a
+ * relatively small and/or low cardinality set of things that are interesting
+ * (with perhaps many more uninteresting things that never populate the
+ * filter).
+ */
+bloom_filter *
+bloom_create(int64 total_elems, int bloom_work_mem, uint32 seed)
+{
+ bloom_filter *filter;
+ int bloom_power;
+ int64 bitset_bytes;
+ int64 bitset_bits;
+
+ /*
+ * Aim for two bytes per element; this is sufficient to get a false
+ * positive rate below 1%, independent of the size of the bitset or total
+ * number of elements. Also, if rounding down the size of the bitset to
+ * the next lowest power of two turns out to be a significant drop, the
+ * false positive rate still won't exceed 2% in almost all cases.
+ */
+ bitset_bytes = Min(bloom_work_mem * 1024L, total_elems * 2);
+ /* Minimum allowable size is 1MB */
+ bitset_bytes = Max(1024L * 1024L, bitset_bytes);
+
+ /* Size in bits should be the highest power of two within budget */
+ bloom_power = my_bloom_power(bitset_bytes * BITS_PER_BYTE);
+ /* bitset_bits is int64 because 2^32 is greater than UINT32_MAX */
+ bitset_bits = INT64CONST(1) << bloom_power;
+ bitset_bytes = bitset_bits / BITS_PER_BYTE;
+
+ /* Allocate bloom filter as all-zeroes */
+ filter = palloc0(offsetof(bloom_filter, bitset) +
+ sizeof(unsigned char) * bitset_bytes);
+ filter->k_hash_funcs = optimal_k(bitset_bits, total_elems);
+ filter->seed = seed;
+ filter->bitset_bits = bitset_bits;
+
+ return filter;
+}
+
+/*
+ * Free Bloom filter
+ */
+void
+bloom_free(bloom_filter *filter)
+{
+ pfree(filter);
+}
+
+/*
+ * Add element to Bloom filter
+ */
+void
+bloom_add_element(bloom_filter *filter, unsigned char *elem, size_t len)
+{
+ uint32 hashes[MAX_HASH_FUNCS];
+ int i;
+
+ k_hashes(filter, hashes, elem, len);
+
+ /* Map a bit-wise address to a byte-wise address + bit offset */
+ for (i = 0; i < filter->k_hash_funcs; i++)
+ {
+ filter->bitset[hashes[i] >> 3] |= 1 << (hashes[i] & 7);
+ }
+}
+
+/*
+ * Test if Bloom filter definitely lacks element.
+ *
+ * Returns true if the element is definitely not in the set of elements
+ * observed by bloom_add_element(). Otherwise, returns false, indicating that
+ * element is probably present in set.
+ */
+bool
+bloom_lacks_element(bloom_filter *filter, unsigned char *elem, size_t len)
+{
+ uint32 hashes[MAX_HASH_FUNCS];
+ int i;
+
+ k_hashes(filter, hashes, elem, len);
+
+ /* Map a bit-wise address to a byte-wise address + bit offset */
+ for (i = 0; i < filter->k_hash_funcs; i++)
+ {
+ if (!(filter->bitset[hashes[i] >> 3] & (1 << (hashes[i] & 7))))
+ return true;
+ }
+
+ return false;
+}
+
+/*
+ * What proportion of bits are currently set?
+ *
+ * Returns proportion, expressed as a multiplier of filter size.
+ *
+ * This is a useful, generic indicator of whether or not a Bloom filter has
+ * summarized the set optimally within the available memory budget. If return
+ * value exceeds 0.5 significantly, then that's either because there was a
+ * dramatic underestimation of set size by the caller, or because available
+ * work_mem is very low relative to the size of the set (less than 2 bits per
+ * element).
+ *
+ * The value returned here should generally be close to 0.5, even when we have
+ * more than enough memory to ensure a false positive rate within target 1% to
+ * 2% band, since more hash functions are used as more memory is available per
+ * element.
+ */
+double
+bloom_prop_bits_set(bloom_filter *filter)
+{
+ int bitset_bytes = filter->bitset_bits / BITS_PER_BYTE;
+ int64 bits_set = 0;
+ int i;
+
+ for (i = 0; i < bitset_bytes; i++)
+ {
+ unsigned char byte = filter->bitset[i];
+
+ while (byte)
+ {
+ bits_set++;
+ byte &= (byte - 1);
+ }
+ }
+
+ return bits_set / (double) filter->bitset_bits;
+}
+
+/*
+ * Which element in the sequence of powers-of-two is less than or equal to
+ * target_bitset_bits?
+ *
+ * Value returned here must be generally safe as the basis for actual bitset
+ * size.
+ *
+ * Bitset is never allowed to exceed 2 ^ 32 bits (512MB). This is sufficient
+ * for the needs of all current callers, and allows us to use 32-bit hash
+ * functions. It also makes it easy to stay under the MaxAllocSize restriction
+ * (caller needs to leave room for non-bitset fields that appear before
+ * flexible array member, so a 1GB bitset would use an allocation that just
+ * exceeds MaxAllocSize).
+ */
+static int
+my_bloom_power(int64 target_bitset_bits)
+{
+ int bloom_power = -1;
+
+ while (target_bitset_bits > 0 && bloom_power < 32)
+ {
+ bloom_power++;
+ target_bitset_bits >>= 1;
+ }
+
+ return bloom_power;
+}
+
+/*
+ * Determine optimal number of hash functions based on size of filter in bits,
+ * and projected total number of elements. The optimal number is the number
+ * that minimizes the false positive rate.
+ */
+static int
+optimal_k(int64 bitset_bits, int64 total_elems)
+{
+ int k = round(log(2.0) * bitset_bits / total_elems);
+
+ return Max(1, Min(k, MAX_HASH_FUNCS));
+}
+
+/*
+ * Generate k hash values for element.
+ *
+ * Caller passes array, which is filled-in with k values determined by hashing
+ * caller's element.
+ *
+ * Only 2 real independent hash functions are actually used to support an
+ * interface of up to MAX_HASH_FUNCS hash functions; "enhanced double hashing"
+ * is used to make this work. See Dillinger & Manolios for details of why
+ * that's okay. "Building a Better Bloom Filter" by Kirsch & Mitzenmacher also
+ * has detailed analysis of the algorithm.
+ */
+static void
+k_hashes(bloom_filter *filter, uint32 *hashes, unsigned char *elem, size_t len)
+{
+ uint32 hasha,
+ hashb;
+ int i;
+
+ hasha = DatumGetUInt32(hash_any(elem, len));
+ hashb = (filter->k_hash_funcs > 1 ? sdbmhash(elem, len) : 0);
+
+ /* Mix seed value */
+ hasha += filter->seed;
+ /* Apply "MOD m" to avoid losing bits/out-of-bounds array access */
+ hasha = hasha % filter->bitset_bits;
+ hashb = hashb % filter->bitset_bits;
+
+ /* First hash */
+ hashes[0] = hasha;
+
+ /* Subsequent hashes */
+ for (i = 1; i < filter->k_hash_funcs; i++)
+ {
+ hasha = (hasha + hashb) % filter->bitset_bits;
+ hashb = (hashb + i) % filter->bitset_bits;
+
+ /* Accumulate hash value for caller */
+ hashes[i] = hasha;
+ }
+}
+
+/*
+ * Hash function is taken from sdbm, a public-domain reimplementation of the
+ * ndbm database library.
+ */
+static uint32
+sdbmhash(unsigned char *elem, size_t len)
+{
+ uint32 hash = 0;
+ int i;
+
+ for (i = 0; i < len; elem++, i++)
+ {
+ hash = (*elem) + (hash << 6) + (hash << 16) - hash;
+ }
+
+ return hash;
+}
diff --git a/src/include/lib/bloomfilter.h b/src/include/lib/bloomfilter.h
new file mode 100644
index 0000000..f46f233
--- /dev/null
+++ b/src/include/lib/bloomfilter.h
@@ -0,0 +1,27 @@
+/*-------------------------------------------------------------------------
+ *
+ * bloomfilter.h
+ * Minimal Bloom filter
+ *
+ * Copyright (c) 2017, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ * src/include/lib/bloomfilter.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef _BLOOMFILTER_H_
+#define _BLOOMFILTER_H_
+
+typedef struct bloom_filter bloom_filter;
+
+extern bloom_filter *bloom_create(int64 total_elems, int bloom_work_mem,
+ uint32 seed);
+extern void bloom_free(bloom_filter *filter);
+extern void bloom_add_element(bloom_filter *filter, unsigned char *elem,
+ size_t len);
+extern bool bloom_lacks_element(bloom_filter *filter, unsigned char *elem,
+ size_t len);
+extern double bloom_prop_bits_set(bloom_filter *filter);
+
+#endif
--
2.7.4
--
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